The present invention relates to a technique for making practicable a holographic memory recording system having a large capacity and a data protection function, and more specifically, to a reflection holographic memory device being optimally consistent with rotation of a disk recording medium and radial movement of the disk recording medium (mechanism for operating an optical pick-up system).
At present, due to rapid development of optical communication technologies, computers, and imaging equipment, a numerous amount of information can be obtained from the Internet and high-definition broadcasting, etc. To record and store this enormous amount of information, in addition to hard disks using magnetic information recording, the usefulness of external recording media for the terabyte class has increased. In addition, limitations on optical memory technology according to reductions in the wavelength of light sources is on the horizon, while research on optical memories is advancing to multilayer and three-dimensional applications. Under these circumstances, holographic memories which enable parallel reading of image data have been researched domestically and overseas as next-generation recording media which realizes a recording capacity of more than 1 terabyte by using three-dimensional space of the media as well as high-speed reading and transfer of 10 gigabytes per second as a data transfer rate.
Conventionally, as holographic memory recording systems, there are available a transmission hologram and a reflection hologram. For multiplex recording of a transmission hologram, an angle multiplex recording system, a spatial shift multiplex recording system, and a random-phase modulation multiplex recording system, etc., have been made practicable, and on the other hand, for multiplex recording of a reflection hologram, only the angle multiplex recording system has been made practicable.
Under these circumstances, most of the systems made practicable are for the transmission hologram (for example, refer to Japanese Patent Application No. H11-88098).
Concerning holographic memories, the inventors of the present invention reported the results of research on optical memories having a recording data protection function by encrypting an original image based on the optical encryption technique, that is, the world's first random-phase modulation technology announced by the inventors (For example, refer to Encrypted Optical Memory System Using Three-Dimensional Keys in Fresnel Domain (O. Matoba and B/ Javidi, Opt. Lett., 24, 762-764 (1999)) and New Optical Encryption Technology (O. Matoba, et. al., Opt. 29 (2000))).
Furthermore, a system using a random-phase modulation mask in an arrangement of a transmission hologram is also known (for example, refer to U.S. Pat. No. 5,716,691).